Lay onion, tomatoes, garlic on a baking sheet and broil – turning occasionally – until tomatoes skins are blistered and charred. This should take 15-20 minutes. Remove “paper from garlic.

Remove seeds, stems and ribs from chilies. Heat a heavy skillet or “comal” and toast chilies. Using tongs, press chilies down on hot surface for a few seconds and turn until all sides are toasted, but pliable. Remove to a bowl and pour hot water over them. Leave the chilies to soak for 20 minutes.

Drain chilies and discard soaking liquid. Combine chilies with roasted onions, tomatoes, and garlic in a blender. Add oregano and puree until very smooth. Pour puree into a soup kettle and add 6 cups chicken broth. Bring to a boil and then let simmer for 40 minutes.